It has been prevalent to increase wear resistance of a sliding member such as a piston ring by nitriding and hardening a surface (a sliding surface) of a base member made of a steel material such as SUS440B, SWOSC-V, SUP and the like and then applying a hard coating to the surface. Examples of the hard coating include amorphous carbon called diamond-like carbon (DLC). A structure of the DLC, in essence, is a mixture of diamond bond (SP3 bond) and graphite bond (SP2 bond) as carbon bond. Having hardness, wear resistance, thermal conductivity, chemical stability that are similar to diamond as well as solid lubricity similar to graphite, the DLC is suitable as a protection film of a sliding member of, for example, an automobile part.
However, a piston ring for a downsizing turbo charged engine with improved fuel efficiency for environmental conservation is used in a very severe sliding environment with high temperature and a high surface pressure. As such, conventional piston rings reach high temperature, accelerating wear thereof. In order to effectively dissipate heat in a combustion chamber from a piston to a cylinder wall surface via the piston ring, the piston ring needs to have high thermal conductivity. Therefore, materials having high thermal conductivity have been used for a base member to ensure wear resistance.
There are various hard coatings that have been developed. For example, patent literature PLT1 set forth below describes a DLC coating applied to a sliding side surface of a sliding member. This DLC coating includes at least two films, a lower film and an upper film, both made of the DLC. The lower film has a hardness of 20 GPa to 45 GPa and Young's modulus of 250 GPa to 450 GPa, while the upper film has a hardness of 5 GPa to 20 GPa and the Young's modulus of 60 GPa to 240 GPa.